| Abstract: | Pituitaries removed from Tilapia mossambica adapted either to sea water or fresh water were incubated for 3 or 6 hours in isosmotic or hyposmotic medium. Activation of prolactin cells from pituitaries incubated in hyposmotic medium was indicated by frequent exocytosis and by proliferation of cellular organelles. These pituitaries also showed a reduction in prolactin content and an increase in the amount of bioassayable prolactin in the medium. Incorporation of tritiated leucine into prolactin was least in prolactin cells from seawater Tilapia incubated in isosmotic medium and greatest in those of freshwater Tilapia incubated in hyposmotic medium. These observations indicate that in Tilapia, both synthesis and secretion of prolactin may be stimulated directly by dilution of the medium bathing the pituitary. On the other hand, there were no ultrastructural changes in the presumptive growth hormone cells incubated for 3 or 6 hours in the hyposmotic medium. Hence, these cells do not appear to play a major osmoregulatory role. |
